The invention is generally related to the field of high viscosity materials for semiconductor processing and more specifically to photolithography using high viscosity materials.
High viscosity materials are applied to semiconductor devices for the purpose of buffer dielectric of photoresist coating. The term high viscosity refers to materials with greater than 1000 centipoise. These viscous materials are difficult to coat with good thickness uniformity. Thickness uniformity becomes more difficult as the diameter of starting wafers gets larger. The centrifugal on the puddle of the material causes internal shear forces that disrupt the flow of fluid. It is difficult to uniformly move the material from the center of the wafer to the edges.
A first method of spin coating, uses a center dispense with high spin speeds. Normally, a high rate of dispense is combined with high RPM (revolutions per minute) spin speeds to force the material from the center dispense point to the edge of the wafer. High viscosity materials have a limited dispense rate typically around 0.5 ml/second maximum. As the dispense RPM increases, the material 12 has a tendency to xe2x80x9cball upxe2x80x9d(14) at the edges and not completely cover the wafer 10, as shown in FIGS. 1A and 1B. If a lower dispense speed is used a xe2x80x9cdomexe2x80x9d profile in resist thickness results, as shown in FIG. 2.
Another method of spin coating uses a swirl dispense method. In this method, a dispense arm is scanned across the wafer as the wafer is rotated. After nearly one revolution, the arm is moved towards the center a precise amount. Another revolution is performed and the arm is moved in again. The dispense is finished at the center and the water rotation is stopped. The wafer is then spun until the material is somewhat uniformly distributed. The wafer is then cast to final thickness. Problems with this method include: (1) timing difference between the pump spin motor and dispense arm, (2) variations in the swirl caused by batch to batch viscosity differences, and (3) high maintenance is required.
The invention is a method for spin coating high viscosity materials. Two dispense steps are used. The first dispense step dispenses a small amount of high viscosity material at the center of the wafer. A high-speed wafer rotation spreads the material to form a thin layer to prime the surface of the wafer and lower the surface tension without regard to uniformity. A second dispense step occurs at lower RPM and coats the wafer more uniformly.
An advantage of the invention is providing a simple method for spin coating a high viscosity material more uniformly across a wafer surface.
This and other advantages will be apparent to those of ordinary skill in the art having reference to the specification in conjunction with the drawings.